1. Field of the Invention
This invention lies in the field of making coated tempered glass panes, particularly glass panes having reduced transmission of solar radiation.
2. Prior Art
Glass panes having a surface coating of a metal or metal alloy are used in the building sector and in vehicle glazing for reducing the transmission of the uncoated glass carrier in certain spectral ranges. This is done e.g., in order to subdue the light and/or obtain a solar control effect. Metals or metal alloys from elements with atomic numbers 22 to 28 of the periodic table are preferably used for the metal coating, if it is wished to obtain glass panes in which there is no colour change due to the coating in transmission and in reflection. Generally standard soda-lime-silica glass is used as the glass carrier and can be additionally in mass, tinted as is the case with bronze, gray blue and green glass. This mass tinting already leads to a basic solar control effect, which is reinforced by the coating. Particularly in the case of glazing systems for motor vehicles, frequent use is made of green tinted glass, which has a good solar control effect, in conjunction with high light transmittance. In such cases, particular interest is attached to mixed glazing systems, some of the panes being additionally coated. Thus, for example, the windscreen and furthest forward side windows, which are subject to high legal requirements regarding the minimum light transmittance, are made from green mass tinted glass. For the rear part of the vehicle, where lower light transmittance values are accepted, use is made of panels which are mass tinted and additionally coated in order to increase the protection against the solar radiation. It is particularly important in such mixed glazing systems to use a coating without any colour distortion in reflection and transmission, so that viewed from the interior there should be no disturbing colour difference between the individual panes. The same applies to the colour impression of the vehicle when viewed from the outside.
In many applications, including those described hereinbefore, it is necessary to thermally toughen the glass carrier, which e.g., takes place for increasing the mechanical stability, for preventing heat cracks and for reducing injury risks in the case of the glass breaking. Toughening is carried out by heating the almost exclusively used soda-lime-silica glass panes in air to a temperature above the glass transformation point, followed by rapid cooling. The temperatures required for toughening are in the range 580.degree. to 680.degree. C. and preferably 600.degree. to 650.degree. C. The same temperature range is also required if the glass panes which come flat from the glass manufacturing process undergo a bending process. Hitherto the application of said colour-neutral metal coatings has visually taken place after the toughening and/or bending process and the cooling of the panes, use generally being made of vacuum coating processes, where the glass panes are coated in vacuum chambers at very low pressure and at temperatures near room-temperature.
This procedure of applying the coating after the toughening and/or bending process has numerous disadvantages compared with a procedure in which initially the coating is applied and then the toughening and/or bending process is carried out. Thus, in the former case only cut sizes can be coated, because toughened panes cannot be cut. However, with regards to the coating procedure it is much more advantageous to coat standard sizes, particularly the lehr end sizes of glass production by the float process. In the latter case, it is much easier to obtain a uniform coating thickness than with cut sizes with the necessary gaps between the individual panes in the vacuum coating apparatus. In addition, the transfer of such standard sizes through the coating apparatus is much less complicated than in the case of having to transfer individual pieces of different sizes.
Another disadvantage is that as a result of the high temperatures of the toughening and/or bending process impurities on the glass surface frequently form such a firm bond therewith that they cannot be removed during the following surface cleaning and before performing the coating process to the extent necessary for said coating process. They are quasi burned into the glass surface, which leads to a disturbing deterioration of the coating quality.
In the case of coating bent panes, the problem of obtaining an adequate coating uniformity is naturally particularly serious, because due to the curvature of the panes the angles and distances between the glass surface and the coating sources differ over the pane change. In addition, the costs of vacuum coating plants for coating bent panes are much higher than for coating flat panes, because it is necessary for the inlet and outlet locks, as well as the locks between the different coating stations to be much wider than when coating flat glass.
For eliminating the aforementioned difficulties, namely permitting the application of the coating prior to the toughening and/or bending process when using metals or metal alloys of elements with atomic numbers 22 to 28, it has already been proposed (German patent application No. P 35 44 840.7-45), prior to the toughening and/or bending process, to apply to a substantially flat glass carrier the metal layer with a majority content of a metal or metal alloy from the elements with atomic numbers 22 to 28 of the periodic table and to the side remote from the glass carrier a protective layer of at least one metal oxide or mixed metal oxide which, based on a metal atom of the metal oxide or oxides, has an oxygen deficit x of 0.05.ltoreq.x.ltoreq.0.4 and a thickness of 10 to 100 nm and a composition such that, during the toughening and/or bending process, there is no noteworthy oxygen diffusion in to the metal layer.
It might be obvious for the toughening and/or bending process to follow the application of the coating to replace the above named metals by platinum, iridium or optionally rhodium because of their oxidation stability. However, the process known from French Pat. No. 12 71 584 of providing glass carriers with platinum or rhodium coatings prior to a tempering process and then carry out the latter has not been succesful when used in a method of the type to which the present application is directed. For explanation purposes, it is pointed out that French Pat. No. 12 71 584 describes a procedure in which relatively thick platinum or rhodium coatings, which in transmission and reflection have the desired colour neutrality are treated, by a tempering process in air. It is also possible according to the said French Patent protect the precious metal coating by a layer of an enamel or glaze, which can be burnt in a furnace. If this process of providing the glass carrier with the platinum or rhodium coating and then carrying out a toughening and/or bending process is used with the relatively thin coatings to which the method of the present application is directed, the tempering process leads to disturbing coating modifications. The coating turns cloudy, which can be detected as a disturbing stray light level, e.g., when illuminating with direct sunlight. Therefore such glass panes are unsuitable for solar control purposes in building or vehicle glazing systems. Moreover, particularly in the case of coated glass carriers with a light transmittance above approximately 40%, based on the light transmittance of the uncoated glass carrier, there is an increase in the transmission for the spectral range of solar radiation and therefore a deterioration of the solar control effect due to the tempering process.